In radio transmitters, a transmission signal, i.e. the signal being transmitted, is amplified in a radio frequency power amplifier which amplifies the transmission signal to a level suitable for transmission over an air interface to a radio receiver. The level of the power-amplified transmission signal should be high enough to enable the radio receiver to decode information contained in the transmission signal.
In order to improve the power-efficiency of the power amplifier, a power supply voltage applied to the power amplifier is an amplitude-modulated signal adapted to track an envelope or an amplitude component of a radio frequency transmission signal amplified by the power amplifier. FIG. 1 illustrates a radio transmitter arrangement comprising a power amplifier 102 and a power management device 100 providing the power amplifier 102 with a power supply voltage Vpa. The power management device 100 receives a constant voltage Vbat from a battery, for example, and a reference signal Vref as inputs. The reference signal Vref may represent a transmission signal to be power amplified and transmitted and, accordingly, contain envelope or amplitude information of the transmission signal. The reference signal Vref may be a modulated information signal. The power management unit 100 may track the envelope (or power) of the transmission signal and output the power supply voltage accordingly.
A critical property of the power management device 100 is the bandwidth of the device 100. This bandwidth will determine, how accurately the power supply voltage Vpa follows the reference signal Vref and, thus, the efficiency of the power amplifier. Bandwidth requirements typically depend on telecommunication standards and transmitter architectures. In systems using Gaussian Minimum Shift Keying (GMSK), such as GSM, a radio frequency (RF) envelope is constant and, hence, there is no need for amplitude modulation of the power supply voltage. The dynamic requirements are set by the time in which the power supply voltage must be ramped-up and down between transmission slots. The required bandwidth is quite low, e.g. less than 100 kHz.
In EDGE (Enhanced Data Rates for GSM Evolution), in case of polar transmitter structures, the bandwidth should be higher than 1-1.5 MHz. In Wideband Code Division Multiple Access (WCDMA) systems, the strictest requirement would in the case of a polar transmitter structure, in which the bandwidth should be higher than 16 MHz. Regarding upcoming systems, e.g. 3.9 G, the understanding is that the bandwidth requirement will be in the order of 20 MHz in case of envelope tracking.
To cope with these requirements in terms of bandwidth, while having good power efficiency, a “parallel-hybrid” amplitude modulator has been proposed (U.S. Pat. No. 7,058,373). In the parallel-hybrid topology, a switching part 202 is connected in parallel with a linear part 200, as illustrated in FIG. 2. The switching part 202 is slow, but can process a large power levels (high-level currents) with good efficiency. The switching part 202 typically processes a frequency interval 0 . . . F1, where F1=1 MHz, for example.
The linear part 200 is fast, but has a low efficiency. The linear part 200 processes a frequency interval F1 . . . F2, where F2=20 MHz, for example. In practice, there is a gradual transition band around frequency F1, where the power processing is handed over from the switching part 202 to the linear part 200.
In the parallel-hybrid amplitude modulator, the bandwidth is set according to the bandwidth of the linear part. Hence, it is possible to achieve large processing bandwidths. The overall efficiency depends on how much power must be processed by the switching part and how much by the linear part or, in other words, by the harmonic content of the reference signal Vref that must be tracked.
A problem associated to the parallel-hybrid topology is how to ensure the correct current sharing between the switching part and the linear part. In other words, how to control the two parts such they process only the frequency ranges that they were intended to? Ensuring correct current sharing is needed to maximize the efficiency because, as mentioned earlier, the efficiency of the linear part is low. The problem is similar regardless of whether a radio transmitter comprising the parallel hybrid amplitude modulator utilizes an envelope tracking power management device or a polar transmitter structure.